The role of proprioceptive reflexes during active movement is still a matter of controversy despite decades of study. Arguments have focused on what mechanical variable (e.g., muscle length, force) the central drive tries to control during movement. The proposed research addresses the question of what does the central drive in fact accomplish, i.e., what combination of mechanical variables is found to be regulated in postural states and controlled during active movement. It has been claimed, and there is support for this view, that the variable regulated by proprioceptive reflexes is a simple combination of muscle length and force. This hypothesis is often referred to as "stiffness regulation". For measurements on a joint the regulated variable translates into a combination of joint angle and joint torque provided there is no cocontraction of antagonists. However, it is not clear whether the dynamic changes in torque and angle that ensue when a perturbation is imposed on the joint are a reflection of the inertia and damping associated with the joint or reflect, in part, the speed of response of the regulatory system. The first aim is to identify the regulated variable while the joint angle and torque change dynamically in response to perturbation. The second aim is to determine whether or not the combination of mechanical variables that was found to be regulated in postural states is what is controlled during active movement. The third and fourth aims address questions related to the commonly observed contraction of antagonist muscles when the need for such contraction is not apparent. Specifically, what external conditions are sufficient to elicit contraction of antagonist and whether the electromyographic stretch reflex of the agonists is affected. The answers to the questions raised above would provide an understanding of the role of proprioceptive reflexes in the control of movement by healthy subjects. Such an understanding is a necessary prerequisite for delineation of the role these reflexes play in pathological conditions characterized by movement disorders.